Recently, liquid crystal display devices have employed LED backlights including light sources using LED (Light Emitting Diode). Generally speaking, liquid crystal display devices using LED backlights are equipped with a function of changing the luminance of a backlight with a user's preferable luminance based on a user's instruction. Due to individual differences of LEDs in terms of actual hues and light quantities, however, individual backlights may vary in luminance irrespective of the same driving condition. Additionally, LEDs may vary in outputs depending on operating conditions such that light quantities will be reduced in proportion to increasing temperatures. Therefore, it is difficult to stabilize the luminance of a backlight at a user's preferable luminance irrespective of individual differences and operating conditions even when the operation of a backlight is solely controlled based on a user's specified luminance.
To solve the aforementioned problem, engineers have proposed a method of using an optical sensor which is able to measure the light quantity of received light (e.g. Patent Literature Document 1). The optical sensor receives part of the light emitted from a backlight so as to measure the light quantity of light actually emitted from a backlight. A BL (backlight) driver carries out a control operation (e.g. a feedback control) to successively adjust a driving condition for a backlight based on a light quantity measurement obtained from the optical sensor.
In general, the aforementioned BL driver includes a low-pass filter which carries out a stabilization process to eliminate noise from the light quantity measurement input from the optical sensor. Thus, the BL driver achieves stabilized feedback control.